1. Field of the Invention
This invention relates to power-up signal switching circuits and, more specifically, to such circuits capable of relatively low current operation.
2. Brief Description of the Prior Art
To control whether or not the external outputs of a device are enabled during power up (i.e, ramping of the power signal (v.sub.cc) from zero volts to its operating voltage), a signal is required which will switch and release control of the external outputs when the power signal has ramped up to some predetermined voltage level which is lower than the final operating level. To this end, a reliable reference has been required to accurately set the voltage level for the power-up signal switching circuit at which switching will take place. Typically, this has meant use of a voltage divider to provide a reference voltage with operation of the voltage divider at relatively high current levels, thereby dissipating a relatively large amount of power.
A favored characteristic of CMOS designs and a reason for their widespread use is their extremely low standby current (I.sub.CC) consumption. Prior art power-up signal switching circuits have required current levels which exceed the level preferred in CMOS designs. It follows that prior art power-up signal switching circuits have not been easily made compatible with CMOS circuitry.
A problem with prior art power-up circuits is that they have not shut off the reference portion of the circuit after completion of power-up. Accordingly, the impedance of the reference portion of the circuit has been made very high in the prior art to reduce the current flow. This made the reference signal unstable as a result of the influence of parasitic currents. Also, the reference circuit current could not be reduced sufficiently for use in conjunction with CMOS devices. It is therefore apparent that a power-up switching circuit which is capable of operation at the low current levels compatible with CMOS designs and also capable of stable switching at the prescribed voltage level is highly desirable. This type of circuit requires a sufficiently strong and reliable reference during the power-up phase to switch when the appropriate voltage has been reached and then, after the switching is completed, essentially turn off or have an extremely low current requirement to provide very low to no current flow and low power dissipation. Though such circuitry is highly desirable and has been long sought, no such circuitry has been known to exist in the prior art.